The present invention relates to an economical iron-based brazing alloy composition which allows for more economical fabrication of brazed assemblies such as heat exchangers. The present invention also relates to brazed assemblies with iron-based brazing alloys.
The need to produce brazed assemblies and in particular heat exchangers in a manner which minimizes cost of materials and yet obtains products which match or exceed the properties and capabilities of assemblies made with yesterday's technology has become overly apparent in recent years. Particularly desirable, yet problematic, in the heat exchanger fabrication business is the need to eliminate the large amounts of nickel required for most braze alloy compositions and to replace the nickel with a more economical substitute.
Plate-fin heat exchangers use significant amounts of braze alloy for fabrication by furnace brazing. These alloys typicaly contain greater than 60% nickel. Substitution of iron for nickel could result in significant raw material cost savings making possible savings which may be passed on to the consumer. Substitution is, however, not a routine matter as iron has a higher melting point than nickel, tending to raise brazing temperatures above practical furnace limits and also tending to weaken the parent elements of the heat exchanger. Also, the combination of iron fillers on iron metals, e.g., stainless steels, can lead to severe erosion problems of the parent metal, typically referred to as burn-through.
Accordingly, the present invention contemplates provision of a braze alloy in which the filler is substantially iron with additions of chromium and/or nickel for improved high temperature service, and carbon, boron and silicon for melting point depression. The carbon level is maintained low to prevent erosive burn-through of the parent or structural member when joined by furnace brazing in a protective atmosphere. A brazing temperature of less than 2100.degree. Fahrenheit, although 2200.degree. F. is tolerable, has been achieved while at the same time achieving the desired objective of reduced raw material costs compared to conventional nickel-based filler metals.
Accordingly, the present invention also contemplates provision of a brazed assembly having an iron-based brazing alloy.